The primary mode of heat transfer for several decades has been single-phase cooling due to its low cost and reliability. Very high heat flux dissipation has become critical for miniaturized electronics with increased chip power densities. The use of microchannels for single-phase cooling has been pursued but is limited by the high chip temperature and large pumping power required. The traditional single-phase heat sinks are no longer capable of dissipating the generated heat in such applications. As a result, introduction of innovative thermal management methods is desired to address the demands of future electronic devices. Phase change heat sinks where boiling of a solvent is carried out has the ability to dissipate large quantities of heat. Phase change heat transfer that utilizes latent heat of vaporization is considered to be a solution for removing high heat flux from electronic devices. Research of flow boiling in microchannels has focused on the heat transfer and pressure drop performance of flow boiling in microchannels.
Flow instability, low heat transfer coefficient, and flow misdistribution have resulted in poor performance of two-phase systems. Various techniques have been pursued to avoid high pressure drops or pressure fluctuations. These techniques include artificial nucleation sites, inlet restrictors, and different inlet/outlet configurations. Over the past decades, extensive research efforts have been focused on enhancing boiling heat transfer and mitigating its issues. Most of these studies involve modification of the heated surface to enhance surface roughness, effective heat transfer area, or active nucleation sites. More recently, wick structures are also employed in microchannels to promote liquid delivery to the heated area and avoid dry-outs. Various geometries have been pursued, including: parallel trapezoidal cross section microchannels; parallel diverging microchannel with artificial nucleation sites; and parallel triangular microchannels. Generally, it is held that the use of inlet restrictors, an increase in system pressure, an increase in channel cross-section, a reduction in the number of channels, and a reduction in channel length allow a more stable flow in microchannels. Additional effects have been pursued by the use of: expanding microchannels for lower pressure drop and wall temperature fluctuation; cross-linked microchannels; square parallel microchannels; and microchannel heat sink with structured reentrant cavities. Open microchannels with tapered manifold configuration have been shown to simultaneously increase the heat transfer coefficient and the critical heat flux (CHF), which is the thermal limit of phase change during heating where there is a sudden decrease in the efficiency of heat transfer and overheating of the heating surface. In this manner, relatively high cooling rates without reaching CHF has been observed. However, high cooling rates have only been achieved with high surface superheating, high mass flux, low heat transfer coefficients, and/or low vapor exit quality. Although improvements in the heat transfer characteristics of boiling at microscale have been reported, the nature of the boiling process is not altered and, hence, the problems associated with boiling could not be eliminated.
The dynamics of heat transfer during nucleation process involves three different sub-mechanisms where, during bubble nucleation, micro-layer evaporation is the most effective mode of heat transfer and can dissipate heat fluxes up to 3 times greater than other sub-mechanisms. Flow boiling heat transfer in microchannels displays heat transfer rates with the thin film evaporation that is greater than the average convective boiling processes. However, thin film/micro-layer evaporative modes of heat transfer occur only for a very short period of time (<5 ms) and over small areas; thus the overall boiling heat transfer rate is lower than these modes. Dynamic and static instability issues associated with boiling at microscale and identified random nucleation, sudden growth of bubbles and moving evaporating thin film liquid is the cause of these instabilities and whenever liquid-vapor interfaces are rapidly disrupted, flow instability issues are intensified.
Hence there remains a need to have a heat sink for miniaturized electronics with high chip power densities that display high cooling rates, high heat transfer coefficients, low surface superheat temperatures, high vapor exit quality, and superior coefficients of performance (COP).